In an automobile speed control mechanism, a braking force is usually converted by a hydraulic braking system into a hydraulic pressure of a brake liquid for transmission to each brake unit. With the hydraulic braking system, a force applied thereto from a brake pedal during braking is converted by a master cylinder into hydraulic pressure, which is further converted to hydraulic pressure suitable for the braking force of each of front and rear wheels and then transmitted to a wheel cylinder of each wheel to actuate its brake unit, thus braking the wheel.
For reducing the speed of an automobile, a large braking force is needed; in addition, its deceleration is limited by a value dependent upon an adhesion coefficient between tires and the ground. It is required that the braking force for the front and rear wheels be properly distributed by selecting a maximum oil pressure on a rear wheel brake smaller than on a front wheel brake. It is well-known in the art that an ideal distribution of the braking force for the front and rear wheels is such that the front brake gradually works with an increase in the braking force therefor, that is, the individual braking forces for the front and rear wheels bear such a relation as is represented by a quadratic curve. But the braking forces for the front and rear wheels are usually fixed, or bear a constant relation to each other, and a variety of hydraulic pressure control valves are employed for achieving such an ideal distribution of the braking force.
With such hydraulic pressure control valves, an input oil pressure from the master cylinder is usually transmitted, as it is, to the brake unit of, for example, the front wheel, whereas, to the brake unit of the rear wheel, the input oil pressure is applied after being reduced by a reciprocating motion of a plunger. These prior art valves are defective as discussed below. FIGS. 1(a) and (b) are a cross-sectional view of a conventional hydraulic pressure control valve and an enlarged view of one portion. As shown in FIG. 1, a valve proper 7 has formed therein brake liquid inlet ports 1 and 3 and outlet ports 2 and 4. A liquid passage from the inlet port 1 to the outlet port 2 constitutes a first hydraulic pressure passage leading to each brake unit of the front wheel, for instance, and a liquid passage from the inlet port 3 to the outlet port 4 constitutes a second hydraulic pressure passage leading to the brake unit of the front wheel, for example. In the central portion of the valve proper 7 is made a stepped hole 12a, in which a stepped plunger 5 is provided so that it may freely reciprocate. One end 5a of the plunger 5 partly extends into the first hydraulic pressure passage separated by a y-seal 6 from the second hydraulic pressure passage.
The other end 5b of the plunger 5 lies in the second hydraulic pressure passage, and a valve flange 5c of a larger diameter than that of the other end 5b is formed between the one end 5a and the other end 5b of the plunger. Further, a spring receiving flange 5d is formed in contact with the valve flange 5c, defining therebetween a circular space 5e. A spring 7a is supported by the spring receiving flange 5d and, by a plug 12 threadably engaged with the valve proper, the spring 7a is urged toward the first hydraulic pressure passage, resulting in the plunger 5 being urged by the resiliency of the spring 7a toward the first hydraulic pressure passage.
Between the two flanges 5c and 5d, that is, in the circular space 5e, is disposed a lip seal 23 made of a resilient material. The lip seal 23 has its outer peripheral surface making contact with the interior surface of the stepped hole 12a and its inner peripheral surface projecting into the circular space 5e between the two flanges 5c and 5d as depicted in FIG. 1(b). The lip seal 23 has formed integrally therewith semi-spherical projections 23a at predetermined intervals on the side of the spring receiving flange 5d. During normal drive operation, as shown in FIGS. 1(a) and (b), the projections 23a of the lip seal 23 are in contact with the spring receiving flange 5d and the brake liquid flowing into the stepped hole 12a from the inlet port 3 is drained out from the outlet port 4, passing through gaps defined between adjacent ones of the semi-spherical projections 23a. In the second hydraulic pressure passage, since the cross-sectional areas of the plunger 5, i.e. its other end 5b and the valve flange 5c are selected different from each other, the brake liquid having entered from the inlet port 3 is drained out, with its hydraulic pressure regulated by the engagement of the lip seal 23 with the outer peripheral surface of the valve flange 5c. That is to say, the plunger 5 lies between the first and second hydraulic pressure passages and reciprocates in dependence upon the force of the spring 7a and the hydraulic pressure of the brake liquid in the first hydraulic pressure passage, by which the hydraulic pressure of the brake liquid is regulated in the second hydraulic pressure passage. Between the first and second hydraulic pressure passages is disposed the Y-seal 6 and, during normal operation, the plunger 5 is urged toward the second hydraulic pressure passage. Accordingly, when the first hydraulic passage fails, the pressure on the plunger 5 decreases by that and the the pressure regulated in the second hydraulic pressure passage increases, resulting in the hydraulic pressure on the side of the outlet port 4 to be regulated as shown in FIG. 3, line (b). The Y-seal 6 is always exposed to the hydraulic pressures in the first and second hydraulic pressure passages and, in addition, the plunger 5 reciprocates, so that the Y-seal 6 held in contact with it is liable to be broken.
The present invention has for its object to cure the abovesaid defect; in concrete terms, the invention is to provide a hydraulic pressure control valve designed so that during normal operation no external forces are applied to the plunger, and that upon failure of either one of the hydraulic pressure passages leading to the brake units of the front and rear wheels, sliding movement of the plunger is inhibited, thereby to hold the hydraulic pressure regulating mechanism inoperative.